Laser disks such as CD ROMs have become the preferred recording medium for audio music and/or computer program distribution. More recently, Digital Video/Versatile Disks (DVD) have been introduced that are predicted to become the preferred choice for recording everything from full-length motion pictures to computer games.
One advantage of a DVD over CD ROM is DVD's superior data storage capacity. The current industry-wide DVD Specifications for Read-Only Disk (published by and obtainable from Toshiba Corporation of Tokyo, Japan; here in after referred to as “DVD Specifications”) support at least the following formats (listed with the corresponding storage capacities for a 12 cm DVD ROM disk):
FormatCapacitySingle sided4.7GBytesDouble sided9.4GBytesSingle sided, dual layered8.54GBytesDouble sided, dual layered17Gbytes
From the above, it is clear that the storage capacities of DVD disks improve dramatically over the storage capacity of 0.65 GByte available on the current CD-ROM. In addition, other optical disks with more than 4 layers of tracks are also being produced.
A laser or an optical disk such as a DVD disk is made up of a reflective metal foil layer such as aluminum encased in a clear substrate. Data are stored on the foil as a series of tiny pits formed in a continues tight spiral on the disk. The pits are formed in the foil by first stamping a disk made of polycarbonate, acrylic, polyolefine or similar material, using a master stamper in a pressing or injection molding process. The surface of the injection molded disk is then metalized with, for example, aluminum metal in a sputtering process to form the metal foil containing the pits. The pits provide the necessary reflective surface so that a laser beam from a DVD player may be reflected to indicate the data pattern on the disc.
As mention above, there are at least 4 different formats currently contemplated for DVD. In the case of a single-sided DVD, the stamped disk is joined to a dummy substrate. For a double-sided disk, two halves, each with a respective metal foil layer containing data, are bonded back to back.
In addition, as defined by the DVD Specifications, two layers of data are possible on each side of a disk. A single-layer disk has only one track per side on a disk, whereas a dual-layer disk has two tracks per side of a disk. A dual-layer disk has both a Layer 0 track closer to the reading surface, and a Layer 1 track, away from the reading surface, as shown, for example, in FIG. 1A. Typically, a dual-focus optical system is employed so that the data on either Layer 0 or Layer 1 on a dual-layer disk can be read. Layer 0 is purposely made partially transparent so that a laser in the optical system can focus on Layer 1 through Layer 0.
FIG. 1B shows the disk structure of a known double-sided, dual-layer disk. The exemplary disk has a total of 4 reflective layers (two on each side of the disk). To read a dual-sided disk, a user either manually flips over the disk or the DVD player use a motor mechanism to automatically flip the disk or the laser.
Since all the laser disks from a single manufacturing run are stamped from the same stamping master, their contents are identical to each other and to the stamping master. It has been recognized that it may be desirable to be able to put certain data (e.g., serial number or encryption key, etc.) unique to each disk or program on the disk so that the disk or the program contained within can be identified. This would allow, for example, a company to be able to offer pay-per-view capability, to prevent piracy, or to remotely activate a computer program.
For example, to offer a pay-per-view service using a DVD disk, a company may decide to give out DVD disks containing a movie program at very low cost and then to charge viewers based on the number of times the movie is played. This pay-per-view application can be accomplished, for example, by having an individualized serial number associated with each individual disk or program being distributed. A DVD player can be built that can interrogate and retrieve this serial number, if the disk is played by the player. This serial number can then be transmitted to the program provider's billing center via a built-in modem, for example, in the DVD player, so that a charge can be billed to the household where the DVD player resides.
In order to support this type of application, the current DVD Specifications define an area on a DVD disk that allows a manufacturer to etch, by a laser, information onto each disk during the post-stamping production. (See, for example, DVD Specifications for Read Only Disk/Part 1, Physical Specification Version 1.0, Annex K).
This “Burst Cutting Area” (BCA) is specified to be located between an inner circumference of 22.3 (+0/−0.4) mm to an outer circumference of 23.50 (+/−0.05) mm from the center of the center hole of a disk, as shown in FIG. 2. BCA is specified in the DVD Specifications only for a single-sided disk, either with single or dual layers. For a dual-layer disk, the DVD Specifications require that the BCA to be on layer 1 of the single-sided disk.
According to the information to be encoded, stripes in a bar-code like shape are formed by partially removing an aluminum reflective layer of a disk using a converged laser beam. When the stripped part of the BCA is reproduced with an optical laser, the amount of reflected light drops to near zero. When compared to normal program pit signal, the BCA signal level is larger in amplitude and longer in cycle. Therefore, the BCA signal is easily distinguished from a DVD program signal by passing the signal through a simple low-pass filter.
A maximum of 2000 stripes in bar code form may be recorded along the circumference of the disk. Maximum of 188 bytes are currently allowed in the BCA strip, which in the bar-code like encoding reaches a rotational angle of 300 degrees along the circumference of the disk, as shown in FIG. 2.
A laser system capable of encoding individualized information on a disk conforming to the BCA requirement of the DVD Specifications is described, for example, in an article entitled “BCA Recording Technology for Adding Individual Information on DVD ROM Disk”, published in National Technical Report of Japan, Vol. 43, No. 3, dated June, 1997. The system employs a CW-Q switch type YGA laser with wavelength of 1.06 um to encode data in a BCA area.
Similarly, D1, EP 0 802 527 A1, discloses an optical disk having a first recording area and a BCA area encoded with identification data. The data in the BCA is used to decipher data recorded on the first encoding area. Nowhere in D1, however, suggests that the optical disk may have different sides or layers or that more than one BCA area may be used on the disk.
In summary, current DVD Specifications as well as the above mentioned article and D1, do not teach or suggest that multiple BCA areas may be included on a single disk, or that a BCA may be applied to a dual-sided disk, or that each layer or each side of a recording medium may have a unique BCA.